Re: Receive Birdies

VK3HN
 

Mike

I built a Progressive Receiver (W7ZOI) last year with si5351 and stayed with the original dual conversion frequency mixing scheme, a first (tuneable) IF of 3.5-4MHz, second IF 9MHz.  I used all 3 si5351 clocks, first for VFO, second for heterodyne osc, 3rd for BFO.  It was riddled with birdies, unusable.  First I built a crystal BFO and buffer, eliminating the need to use the third si5351 clock.  Made almost no discernible difference.  Finally I ditched the dual conversion scheme and reconfigured it into a single conversion superhet with high side VFO.  Problem fixed.   Well almost.  There are still a few odd birdies, but just a handful, all are well below the usual received signals.  I now use this receiver frequently on 160 to 40m and it works well.  has some gain distribution issues but that's a different problem.  

I suspect the clocks are rich in harmonics and that by placing f-VFO above f-IF they no longer bother me. 

My rx has tight receive BPFs.  It could be shielded better.

I also started scratch building a uBitx and got it receiving, kind of, not really, lost interest for another build, will come back to it one day. 

Regards  Paul VK3HN.  

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